Table of Contents
Introduction
Have you wondered if you go to a clinic or hospital where a doctor or nurse checks your blood pressure with a mercury sphygmomanometer or with an electronic one and writes down your BP as an example, 120/80 mmHg, and you have questioned if it’s a pressure, then why is it not showing in some other units of pressure, such as pascal, bar or N/m²?
If electronic blood pressure measuring devices are available and they are used very much nowadays, then why don’t they measure blood pressure with some other units?
Blood pressure is measured in millimetres of mercury (mmHg) because it originates from the earliest and most reliable method of measuring pressure — the mercury sphygmomanometer. Even though modern devices are digital, we still use mmHg to ensure consistency and accuracy in medical readings.
What is blood pressure?
Blood pressure is the force that blood exerts on the walls of your arteries as your heart pumps it through your body. It’s reported as two numbers — systolic (pressure when the heart beats) and diastolic (pressure when the heart rests). For example, 120/80 mmHg means a systolic pressure of 120 and a diastolic pressure of 80.
Why “mmHg” is Used for Blood Pressure
The term mmHg stands for millimetres of mercury. It refers to how high the pressure in your blood can push a column of mercury in a tube. If your blood pressure is 140 mmHg, that means it can lift mercury by 140 millimetres, or 14 centimetres.
The sphygmomanometer was invented in the year 1881 by Samuel Siegfried Karl Ritter von Basch; a more easily usable version was introduced by Scipione Riva-Rocci in 1896. In 1905, Russian physician Nikolai Korotkov improved this and included diastolic blood pressure measurement following his discovery of “Korotkov Sounds”.
The first blood pressure measuring instrument used a column of mercury to measure the pressure of the flowing blood in arteries. The pressure was calculated how much height of mercury can be raised in the column, till you sto
Even though modern electronic or aneroid (dial-type) devices no longer use mercury, the mmHg unit remains the standard because it allows comparison with historical data and ensures accuracy across all blood pressure measurements.
Why Mercury Was Used
Mercury is a viable choice for a manometer because it is the only metal that is liquid at room temperature. Mercury was chosen for several reasons:
- High density: Mercury is 13.6 times denser than water, meaning even high pressures need only a small column of mercury.
- Stability: Mercury doesn’t evaporate easily or compress, so it gives precise and repeatable readings.
- Visibility: It has a shiny, metallic surface, making it easy to see in the measurement tube.
How to measure blood pressure?
How to measure blood pressure?
About 5-10 minutes
Prepare for Measurement
Sit quietly for 3-5 minutes before measuring. Rest your back against a chair, keep your feet flat on the floor, and avoid crossing your legs. Ensure your arm is supported at heart level and remove any clothing covering your upper arm.
Apply the Cuff
Wrap the blood pressure cuff snugly around your bare upper arm, about one inch above the bend of the elbow. Make sure it is neither too tight nor too loose, and two fingertips can fit under the edge.
Inflate the Cuff
For manual monitors, use the pump to inflate the cuff rapidly to at least 30 mmHg above your expected systolic pressure. For digital monitors, press the button according to manufacturer instructions.
Take the Reading
Slowly release the pressure at about 2–3 mmHg per second. Note the pressure when you first hear the pulse (Korotkoff sound)—this is the systolic pressure. Continue releasing until the sound disappears—this is the diastolic pressure. For digital monitors, read the measurement displayed.
Record the Results
Write down or record your blood pressure reading (systolic/diastolic) and note the time. Wait 1–2 minutes before repeating and take 2–3 readings for accuracy.
Estimated Cost: 1 Free if using personal device; costs vary for monitor purchase.
Supply:
- Blood pressure cuff
Tools:
- Blood pressure monitor (manual or digital)
- Stethoscope (for manual measurement)
- Chair
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